Method of fabricating dark field coaxial ultrasonic transducer

ABSTRACT

A dark field ultrasonic transducer is constructed with an outer annular spherical or conical transducer element and an inner spherical element. The outer annular element is excited and insonifies a small portion of a part surface near a discontinuity or crack with longitudinal waves or with surface waves. The inner dark field element is not focused to be sensitive to either reflected sound or waves reradiated from the surface waves, but detects sound scattered from surface discontinuities such as a crack edge. When surface waves strike a crack edge and restrike it after reflection from the bottom of the crack, two pulses are received and the time delay between them is a measure of crack depth. The crack shape and crack depth profile are determined as the part is scanned. A sphere-cone transducer, the preferred embodiment, is fabricated by stretching thin piezoelectric polymer film over a tool having a ball embedded in a conical surface.

This application is a division of application Ser. No. 934,520 filedNov. 24, 1986 now U.S. Pat. No. 4,760,304.

BACKGROUND OF THE INVENTION

This invention relates to ultrasonic transducers for nondestructiveevaluation and their fabrication, and especially to dark fieldtransducers to inspect a solid part for flaws and cracks.

Several NDE techniques such as eddy current, fluorescent penetrant,magnetic particle, and ultrasonic surface wave, exist for locating nearsurface cracks in parts. With some of these it is possible to measurecrack length and to measure average crack depth. It is desirable, inaddition to determining crack shape, to measure crack depth along thecrack in order to make cost effective part rejection decisions and toprovide information for fatigue predictions. Existing methods of NDE donot provide capability for measuring crack depth profiles, and there isa need to estimate the size of other types of surface flaws.

In the field of optics, dark field illumination is known in which theilluminating beam is a hollow cone of light; the object is viewed with acenter lens which collects scattered light. The principle has not beenapplied to ultrasonic transducers, however.

SUMMARY OF THE INVENTION

An object of the invention is to determine surface irregularities,including crack depth profile, nondestructively using dark field coaxialultrasonic transducers.

Another object is to provide dark field coaxial ultrasonic transducersusing longitudinal and surface wave insonification, and having an innerdark field element to detect ultrasonic scattering resulting from modeconversion at a surface discontinuity or from scattering from thediscontinuity at angles near but not including 180°, i.e. not includingbackscattering.

Another object is the provision of a method of fabricating a sphere-coneembodiment of a dark field transducer from thin piezoelectric film.

The dark field ultrasonic transducers are comprised of only two coaxialtransducer elements, an outer annular transmitting element and innerdetecting transducer element. They are constructed for insonification ofthe surface with longitudinal waves or with surface waves, or with both.The specific embodiments follow from the manner in which reflection,refraction, and mode conversion vary with the angle of incidence of thelongitudinal wave in the fluid. At an angle of incidence greater thanthe shear critical angle (see FIG. 2) sound is totally reflected andneither shear waves or longitudinal waves are transmitted into the solidpart. When the angle of incidence is increased further to the Rayleighcritical angle, a surface wave is generated and propagates along thesolid surface. The surface wave reradiates into the fluid aslongitudinal waves, but there is an apparent shift in the apparent pointfrom which the longitudinal waves leave the surface known as the Schochdisplacement.

These dark field transducers have an outer annular element which isexcited and emits acoustic waves that are incident on the surface of thesolid body at an angle such that reflected sound waves from the surfacedo not impinge on the inner element. The inner transducer element isspherical and has its focal point on or close to the surface of thesolid body and detects sound scattered and reradiated from surfacediscontinuities and cracks while being insensitive to any reflectedlongitudinal waves or to reradiated longitudinal waves from surfacewaves.

A dark field transducer for longitudinal insonification has an outerelement with an angular extent between an inner angle greater than theshear critical angle and an outer angle less than the Rayleigh criticalangle, whereby incident longitudinal waves are totally reflected.

The principal embodiments are dark field transducers for surface waveinsonification. The outer transducer element is spherical or conical.The former has an angular extent from an inner angle greater than theshear critical angle to an outer angle greater than the Rayleighcritical angle; incident longitudinal waves are converted to surfacewaves that propagate inwardly along the surface of the solid body. Theinner and outer elements are confocal or nonconfocal. The sphere-conetransducer configuration has a conical outer element to generate planewaves that are incident on the surface of the solid body at an angleequal to the Rayleigh critical angle.

Another feature of the invention is that the diameter of the surfacewave generation ring is desirably less than or equal to the Schochdisplacement, to prevent significant leakage of surface wave energy backinto the fluid as longitudinal waves.

The invention also encompasses a system to detect the shape and crackdepth profile of cracks in a part, preferably using a surface wave darkfield transducer. Surface waves generated on the part are scattered uponencountering a crack edge, reradiating as a first longitudinal wave andgenerating a surface wave which reflects from the bottom of the crackand returns to the crack edge and reradiates as a second longitudinalwave. The inner dark field transducer element detects the first andsecond longitudinal waves reradiated from the crack edge and generatesvoltage pulses. Means are provided to determine the time delay betweenthese pulses and thus crack depth at locations along the crack length asthe part is scanned.

Another aspect of the invention is a method of fabricating a sphere-conedark field ultrasonic transducer using thin piezoelectric polymer filmsuch as PVDF. A tool with which the thin film is shaped is made byplacing a metal ball at the bottom of a cone in a mold and filling thebore and connecting portion of the cone with material such as epoxy.After curing the epoxy the tool is removed from the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating features of a dark fieldultrasonic transducer system.

FIG. 2 shows the manner in which reflection, refraction, and modeconversion vary with the angle of incidence of the longitudinal acousticwave in the fluid.

FIG. 3 shows a dark field transducer for longitudinal insonification.

FIG. 4 illustrates a non-confocal dark field transducer for surface waveinsonification.

FIG. 5 is a diagram to explain selection of the diameter of the surfacewave generation ring in FIG. 4 to be less than the Schoch displacment.

FIGS. 6 and 7 show alternate embodiments of dark field transducers whichhave spherical elements and generate surface waves.

FIG. 8 shows a sphere-cone dark field transducer for more efficientsurface wave insonification.

FIG. 9 is a cross sectional view of a mold to make a tool with which tomanufacture the sphere-cone transducer.

FIG. 10 is a side view of the foregoing tool.

FIG. 11 illustrates stretching thin piezoelectric polymer film over thetool to form a dark field transducer.

FIG. 12 illustrates scanning a part which has a surface-breaking crack.

FIG. 13 shows a received signal and pulses indicating the top and bottomof a crack in the part.

FIG. 14 illustrates one type of display of the crack shape and crackdepth along its length.

DETAILED DESCRIPTION OF THE INVENTION

The dark field ultrasonic transducers are non-contacting and use anambient fluid, which may be water, about the solid part 15 to beinspected. The dark field transducers consist of two coaxial transducerelements as shown in FIG. 1, an inner transducer element 16 and anannular outer transducer element 17. Both elements have a thin centralpiezoelectric layer and thin metallic electrodes on both surfaces. Theouter annular element 17 is connected to a pulse generator 18 or otherexcitation source, and insonifies a small portion of the part surfacenear a discontinuity or crack with longitudinal waves or with surfacewaves. The inner dark field element 16 is not focused to be sensitive toeither reflected sound or waves reradiated from surface waves. Innertransducer element 16 is connected to a detector, processor and display19, and detects scattered sound. The ultrasonic scattering detected bythe dark field element 16 results from mode conversion at a surfacediscontinuity or from scattering from the discontinuity at angles lessthan 180°. The inner and outer transducer elements 16 and 17 of thistransducer configuration are spherical, coplanar, and confocal. Theelements are focused on the sample surface at the discontinuity or crackedge.

The illustrative embodiments of a dark field transducer in FIGS. 3-8follow from the manner in which reflection, refraction, and modeconversion vary with angle of incidence of the longitudinal wave in thefluid, as shown in FIG. 2. FIG. 2(a) shows a longitudinal wave L,incident normal to the surface, and transmitted into the solid as alongitudinal wave. FIG. 2(b) shows an incident angle less than thecritical longitudinal angle, AL, resulting in transmission of both shearwaves S and longitudinal waves into the solid. In FIG. 2(c). theincident longitudinal wave is at the critical longitudinal angle, AL,where longitudinal propagation in the solid ceases and the transmittedwave has become a shear wave. When the incidence angle is increasedfurther to the shear critical angle, AS, sound is totally reflected asshown in FIG. 2(d). The shear wave does not leave the surface of thesolid. When the angle of incidence is increased further to the Rayleighcritical angle, AR, a surface wave R propogates along the solid surface.As the surface wave propogates in the presence of the fluid, itreradiates into the liquid as a longitudinal wave. This reradiated wave,shown in FIG. 2(e) as a dashed arrow, interferes with the reflectedlongitudinal wave. The result is a lateral shift in the apparent pointfrom which the longitudinal waves leave the surface, Schock displacementD. Refer to M. A. Breazeale et al, Jr. of Applied Physics, Vol. 48,February 1977, pp. 530-537.

The critical angles for a superalloy, Rene 95, and water areapproximately: AL=13.6°, AS=28.8°, AR=31.2°. For many materials there isno more than three or four degrees difference between AS and AR.

A dark field transducer for longitudinal insonification is shown in FIG.3. The inner, spherical, detecting transducer element 20 is limited inangle to Al; the outer, annular, spherical, transmitting transducerelement 21 occupies an angular sector between inner and outer angles A2and A3. When A3 is made less than the Rayleigh critical angle AR thereis longitudinal excitation. When A3 is made less than AR and A2 is madegreater than the shear critical angle AS, there is no propogation intothe solid body 15 and the dark field transducer is sensitive primarilyto surface discontinuities. The two transducer elements 20 and 21 areconfocal, i.e. the focal points are at the same point. To explainfurther, acoustic waves emitted by annular outer element 21 are totallyreflected at the surface of body 15 at an angle larger than AS and thusare not detected by the inner dark field element 20. Longitudinal wavesincident on a surface discontinuity are scattered in many directions andthe scattered sound is detected by inner transducer element 20.Longitudinal waves incident on a crack edge (as in FIG. 4) are scatteredby the crack edge; some of the acoustic energy is detected by innerelement 20 and some travels to the bottom of the crack and back up againto the crack edge where it is scattered so that a satellite pulse isgenerated by the inner piezoelectric element. The time delay between thetwo voltage pulses is a measure of crack depth at that location.

A first embodiment of a dark field transducer for surface waveinsonification is illustrated in FIG. 4. Inner element 22 and outerannular element 23 are both spherical and are coaxial but are notcoplanar. The latter helps to decouple the inner element from the outer.In this case angle A2 is made larger than the shear critical angle AS sothat no rays propogate into the solid part 15, and angle A3 is madelarger than the Rayleigh critical angle AR so that surface waves will begenerated on the solid. The surface waves are indicated by arrows. Theinner and outer elements 22 and 23 are non-confocal. The focal point ofinner element 22 is on the surface of solid part 15, and the focal point24 of outer element 23 is placed a short distance below that of theinner element and the surface of the part. This results in thegeneration of a ring of surface waves which propogate inwardly on thesurface through the focus of the inner element 22. The maximum diameterof the surface wave generation ring is indicated at 25.

The incident surface wave energy is scattered when it encounters a sharpdiscontinuity, like the edge of a surface crack 26, reradiating aslongitudinal waves and generating a surface wave which travels down thecrack and reflects from the bottom of the crack, returns to the cracktip, and reradiates as a longitudinal wave. Two pulses are received bythe inner detecting element 22 from the crack edge, a first pulse whenthe incident surface wave excitation hits the crack edge, and a secondsatellite pulse when the surface wave traveling down the crack and thanback up again strikes the crack edge. The time delay between thesepulses (see FIG. 13) is a measure of crack depth at a particularlocation along the crack length.

Since the surface waves leak energy back into the fluid as longitudinalwaves, it is desirable that their propagation distance be small enoughthat intensity is high but large enough for good decoupling between theinner and outer elements 22 and 23. This is insured by selecting thediameter of the surface wave generation ring 25 to be less than or equalto the Schoch displacement D. The configuration is shown in FIG. 5,where P is the diameter of the ring 25 of surface wave generation and Pis less than or equal to the Schoch displacement D. The ring diameter Pand the Rayleigh critical angle AR determine the distance T between thefocal point of the inner element 22 and the outer element 23 (T=P/2 tanAR).

The dark field ultrasonic transducer may also be employed withcontinuous excitation. A more complete analysis of a pulsed transducershows that the inner dark field transducer element 22 responds toseveral excitations. There is cross-coupling, electrical and mechanical,with the initial pulse from the outer element 23. The reflected soundand waves reradiated from surface waves cause an output even though theinner element is not in focus for these excitations. The two pulsesreceived by the element from the crack edge have been described. In thecase of continuous excitation there is no time separation between thefour received signals. The first two of these, cross-coupling andreflected or reradiated signals, are minimized by the transducer designand may be balanced out by operation in a bridge circuit such as abalanced hybrid. The depth information is then obtained by examining theinterference of the first crack edge signal and the satellite crack edgesignal as frequency is swept.

FIG. 6 illustrates another embodiment of a dark field ultrasonictransducer for surface wave insonification. Inner and outer transducerelements 27 and 28 are spherical, non-confocal, and coplanar. The outertransducer element 28 has an angular extent between an inner anglegreater than the shear critical angle of the solid and an outer anglegreater than the Rayleigh critical angle. Inner element 27 is focused onthe surface of part 15 and outer annular element 28 has an offset focalpoint 29 close to the surface of the part. When the outer element 28 isexcited and emits acoustic waves, surface waves 30 are generated on thesurface of the part. The diameter of the surface wave regeneration ringis preferably less than the Schoch displacement.

FIG. 7 shows a confocal spherical embodiment of the dark fieldtransducer which generates surface waves Inner and outer transducerelements 31 and 32 are spherical and coplanar, and have a common focalpoint 33 which is close to the surface of part 15 so that a ring ofsurface waves 34 is generated and its maximum diameter is again madeless than the Schoch displacement. An advantage of the FIG. 7 transducercompared to FIG. 6 is that the surface wave trains propagating from thecircumference toward the center of the ring are in step andconstructively interfere to maximize the insonification at the center ofthe ring.

Referring to FIG. 8, the preferred embodiment of a dark field transducerto generate surface waves for flaw characterization has a sphericalinner transducer element 35 and a coaxial, annular, conical outerelement 36. The inner element 35 is focused at or just below the surfaceof part 15. Conical outer element 36 is tangent to or coplanar with theedge of inner element 35, and its inner edge typically is at an angleequal to or greater than the Rayleigh critical angle AR. Conical outerelement 36 transmits plane waves; these are incident on the surface ofpart 15 at an angle equaling or exceeding AR so that the incidentacoustic waves are converted to surface waves 37 which propogateinwardly toward the focus of inner element 35. This acoustic lensconfiguration more efficiently converts the incident beam into surfacewaves. All sound waves that create Rayleigh waves stay in phase and thisdark field transducer fully uses the outer transducer element 36. Thediameter of the surface wave generation ring, measured from the pointwhere the ray from the outer edge of conical element 36 is incident onthe sample surface, is no greater than the Schoch displacement.

A method of fabricating such a sphere-cone dark field transducer fromthin piezoelectric polymer film is explained with reference to FIGS.9-11. High frequency ultrasonic transducers, greater than 10 MHz and upto 45 MHz, are made with PVDF (polyvinylidene difluoride) piezoelectricelements. The film is very thin and coated with a very thin layer ofmedal. The polymer film is shaped and given the desired curvature usingthe special tool shown in these figures.

A Teflon® mold 38 has a cylindrical bore 39 leading to a conical cavity40. A metal ball bearing 41, say 1/4" in diameter, is placed at thebottom of the cone and the cylindrical bore and connecting portion ofthe conical cavity are filled with a filler material 42 such asthermosetting epoxy resin. After curing and hardening the epoxy, themold 38 is removed leaving the tool 43 which has the metal ball embeddedin a conical epoxy surface. The end of tool 43 has the desiredsphere-cone shape. Metal is selectively removed from the thin coatedPVDF film 44 to delineate the electrodes and inner and outer transducerelements, and the film is stretched over the end of tool 43 to shape thetransducer and give it the desired curvature. A backing material is nowapplied to one surface of the transducer.

FIG. 12 has a top view of the dark field transducer for surface waveinsonification having the spherical inner element 35 and conical,annular outer element 36, and shows scanning the part 15 to inspect itfor surface irregularities and cracks. The shape of the crack isdetermined and crack depth is measured along the crack length; thisinformation is used in making part rejection decisions and for fatiguepredictions. As the dark field transducer is scanned, say, in the Xdirection at a given Y coordinate, a first pulse is received by innerdetecting element 35 when the incident surface wave excitation hits theleft crack edge, and a second satellite pulse when the surface wavetraveling down the crack and then back up strikes that crack edge. Thereceived signal is seen in FIG. 13. Crack depth is derived from the timedelay t between these two pulses. As the transducer is moved to theright, the right hand crack edge is detected and there are two morevoltage pulses in the received waveform.

The crack shape is determined by plotting the location of the largervoltage pulse in each set of two pulses. One type of display isillustrated in FIG. 14 where the shape of the crack at the surface isdepicted at 26'. Time delays are plotted vertically and indicated byarrows 45 at various locations along the crack length. The crack depthprofile is the envelope of these arrows.

While the invention has been particularly shown and described withreference to several preferred embodiments, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made without departing from the spirit and scope ofthe invention.

The invention claimed is:
 1. The method of fabricating a dark fieldultrasonic transducer having a spherical inner element and a coaxial,annular, conical outer element, comprising:providing a mold having acylindrical bore leading to a cone; placing a metal ball at the bottomof said cone; filling said bore and a connecting portion of said conewith a filler material; curing said filler material and removing theresulting tool, which has said ball embedded in a conical surface, formsaid mold; selectively removing metal from a thin piezoelectric polymerfrom coated with a layer of metal to delineate said inner and outerelements, and shaping and curving said thin film with the end of saidtool.
 2. The method of claim 1 wherein said filler material is epoxyresin and said piezoelectric polymer film is polyvinylidene difluoride.